Optimize scan process with quick/deep modes and archive tag merging

Quick scan (default):
- Pre-load all ImportTask records for O(1) duplicate checks
- Batch task creation commits (50 at a time)
- Cache import settings (5-min TTL)
- Skip pHash comparison for speed

Deep scan (on-demand):
- Full reprocessing of all files
- pHash similarity detection
- Optional thumbnail regeneration
- Optional sidecar re-application

Archive tag merging:
- Add archive tags to existing images when duplicates found
- Works for both hash and pHash duplicate detection

Also fixes:
- Add missing source/post tag icons to gallery templates

app/static/js/bulk-select.js

@@ -555,11 +555,13 @@
         item.appendChild(overlay);
       }
 
-      // Render tags
+      // Render tags with icons
       overlay.innerHTML = visibleTags.map(t => {
         const displayName = getTagDisplayName(t.name, t.kind);
+        const icon = getTagIcon(t.kind);
         const encodedName = encodeURIComponent(t.name);
-        return `<a class="tag-chip" href="/gallery?tag=${encodedName}" title="Filter by ${escapeHtml(t.name)}" onclick="event.stopPropagation()">${escapeHtml(displayName)}</a>`;
+        const prefix = icon !== '#' ? icon + ' ' : '#';
+        return `<a class="tag-chip" href="/gallery?tag=${encodedName}" title="Filter by ${escapeHtml(t.name)}" onclick="event.stopPropagation()">${prefix}${escapeHtml(displayName)}</a>`;
       }).join('');
 
     } catch (e) {